1. Field of the Invention
The present invention relates to a silicon wafer and its manufacturing method. More particularly, the present invention relates to a silicon wafer having a polycrystalline silicon film for use in gettering operations which is formed on the main surface (hereinafter referred to as a "reverse surface") opposite the main surface (hereinafter referred to as a "front surface") on which semiconductor devices are fabricated, as well as to a method of manufacturing the silicon wafer.
2. Description of the Related Art
For various reasons, impurities such as heavy metals are generated during fabrication of a semiconductor device from a silicon wafer, and the thus-generated impurities contaminate the vicinity of the surface of a silicon wafer which will become an active area in the semiconductor device. If the vicinity of the surface of the silicon wafer which will become an active area is contaminated by impurities, the characteristics of the device are degraded, which in turn reduces the proportion of non-defective devices. To prevent such a problem, a technique of intentionally collecting impurities outside of the active area; namely, a gettering technique, is commonly used. A representative method of the gettering technique is the forming of polycrystalline silicon film for use in gettering operations on the reverse surface of a silicon wafer.
According to this conventional method, a polycrystalline silicon layer is usually formed by growing polycrystalline silicon on the reverse surface of the silicon using a LPCVD (Low Pressure Chemical Vapor Deposition) method. The characteristics of the polycrystalline silicon film thus formed as a gettering film by the low pressure chemical vapor deposition method are represented by the stress acting on the silicon wafer, as well as by the gettering capability.
Specifically, the polycrystalline silicon film imposes stress on the silicon wafer having the polycrystalline silicon film formed on the reverse surface thereof. As a result, as opposed to a silicon wafer without a polycrystalline silicon layer, the silicon wafer having a polycrystalline layer becomes warped to a great extent.
Considerable warpage of the wafer presents a problem in the process of fabricating semiconductor devices, and hence there is a demand for a silicon wafer having a polycrystalline film which reduces the stress acting on the silicon wafer.
Particularly, even if a given magnitude of stress imposed on a silicon wafer by the polycrystalline silicon film is identical, the warpage developed in the silicon wafer becomes greater with increasing diameter of the silicon wafer. Associated with a recent tendency of semiconductor devices to be highly integrated with higher accuracy, the diameter of the silicon wafer is steadily increasing. Because of this, it is to be desired that the warpage of the silicon wafer be reduced to as small an extent as possible.
As illustrated in FIG. 5, if an attempt is made to reduce the stress acting on the silicon wafer, the gettering capability of the polycrystalline silicon layer decreases. The relationship between the stress and the gettering capability remains unchanged even with changes in parameters; e.g., the temperature and pressure within a chamber used for low pressure chemical vapor deposition, and a flow rate of SiH.sub.4 (which is reaction gas). For this reason, it has been deemed that the stress acting on the silicon wafer cannot be reduced without decreasing the gettering capability of the polycrystalline silicon film.
As illustrated in FIGS. 3 and 4, when attention is directed to a proportion of a component in the polycrystalline silicon film which have a &lt;220&gt; orientation (hereinafter referred to as the "&lt;220&gt; oriented component"), the gettering capability of the polycrystalline silicon film is improved with an increase in the proportion of the &lt;220&gt; oriented component, and the stress acting on the silicon wafer becomes greater, as well. The reason for this is as follows: Namely, the polycrystalline silicon film is composed of silicon crystal of various grain sizes, and silicon crystal of small grain size contains a large amount of &lt;220&gt; oriented component. The smaller the grain size of silicon crystal, the greater the surface area of grain boundaries. As a result of this, the number of getter sites increases, and hence the gettering capability of the polycrystalline silicon film also increases. At the same time, as the grain size of silicon crystal decreases, the stress produced between grains increases, so that the stress which the polycrystalline silicon film imposes on the silicon wafer increases.
As disclosed in Japanese Patent Application Laid-Open (kokai) No. 4-333238, there already exists a technique which is intended to improve the gettering capability of a polycrystalline silicon film by increasing the amount of the &lt;220&gt; oriented component in the polycrystalline silicon film. As previously described, this technique has a drawback; i.e., an increase in the stress acting on the silicon wafer, which thereby increases warpage of the wafer.